I had a potential customer ask me about the reliability of our repair process. Here is my email response which I thought had some good info:
First - Yes we do still offer the repair, and I apologize for the lack of response to your calls. I am looking at better, more responsive ways to handle this work and correspond. Email is the best way to reach me in particular. Our repairs are handled by my technician, Sebastian Sarmiento. (his email sebastian@firstphasetech.com ) You can also call him 480-967-1100.
Regarding the success rate and reliability - My theory is that almost all the G3 and G4 computers with this problem have solder paste deposition during manufacturing at the root of the problem. When solder paste is squeegee'd through a stencil on to a circuit board, the consistency in paste volume deposited on each of the BGA (video chips) pads, varies. It varies for a number of reasons, squeegee pressure, changes in paste volume on the squeegee blade. If the chip pads have different volumes of paste deposited, this will mean it has different sized solder balls after reflow.
Now... over time when that chip (probably the hottest device in operation) expands and contracts in the Z direction, over time the smaller ball fractures. So, the best thermo-mechanical solution, although the success rate is harder to determine, would be to remove, reball and replace the device. However, this means that the chip will see 5 thermal cycles of approximately 200 degrees C. Some of the ATI video devices are multichip modules, they are essentially a BGA with 2 memory BGA's on top of them. They will not survive 5 thermal reflow cycles in most cases without reflowing (disturbing) the smaller chips on top.
The ATI video chips that are all 1 package have a much higher success rate. Most times we just reflow the device. The success of this reflow depends on the degree of inconsistent paste was deposited during manufacturing. I have some boards that are still in operation now for over 2 years. On some, very few occasions, I have had units die again in a week or two (although I have not heard of this in well over 4 mos).
Anyway - I can share more by phone but have to run. Our site right now has a Google checkout button on the ibook page, or you can use ebay, or just send in the board. Please send in the downloadable form though so we can identofy your board when it arrives. I can get your board out of here the same day it arrives right now. Here is our online info on the repair:
http://www.firstphasetech.com/ibook-repair-g3-g4.html
Thanks
Tom
Friday, June 27, 2008
Saturday, June 21, 2008
Design for QFN Devices
August 03, 2007
QFN Devices
QFN Layout GuidelinesWhile QFN (quad flatpack, no leads) and DFN (dual flatpack, no leads) packaged parts are becoming more and more common in new component releases, they aren t getting much easier to use. The advantages of the form factor are pretty clear. It allows smaller geometries, better grounding and improved thermal properties over other types of surface mount packages.
Most QFNs have a center metal pad on the underside of the part, typically for grounding or heatconduction. It s this center metal pad that makes this form factor so difficult to use. DFN packages are identical, for the purposes of assembly, except that they have pin rows on only two sides of the part.
QFN Float- The middle of the part has a metal contact pad like most QFN packaged parts. Itmay be there for grounding or heat conduction, depending on the specific part. Thefloat that I'm talking about happens when we lay too much solder paste on the PCBfor that center pad. To a small extent, the height of the solder paste deposit is proportional to theaperture in the solder stencil opening (bigger opening = taller deposit). With mostparts, that isn't a problem because either all of the pads are big enough so that thatratio doesn't have a first order impact, or because all of the pads are the same sizeand will be equally impacted. Since the QFN center pad is a much larger opening in the stencil than the signal pad openings, and the signal pad openings are in the 10 - 20 mil or less range, this deposit height to width ratio will have a firstorder impact. When the opening for the center pad on the QFN is too large, the paste deposit in the center will be taller than the deposits on the small signal pin pads. The part high-centers and never gets the opportunity to contact the signal pads. In some cases, the part will tilt a little sideways and contact some of the signal pads but not all. Solder Paste Stencil Typically, the signal pads should have a standoff height of 2 - 3 mils after assembly. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from connecting. To help prevent this, the solder stencil opening should be broken into a series of smaller openings and should cover between 50 and 75% of the pad area. This means that when you lay out your PCB, you need to look carefully at the solder paste layer for your QFN components. If the solder paste layer in the CAD package part library just follows the copper pad pattern or the solder mask opening, you may need to customize the CAD package part library to avoid leading yourself into trouble. To better illustrate the proper way to make your solder paste stencil for QFN parts, I went to our back room and took a couple of photos of good and bad solder paste stencil practices. This is what a just about worst-case stencil would look like. Actual size for this part is 7 x 7 mm. Note how much surface area that the center pad has compared to the row of side cutouts. With most SMT components, it is standard procedure to reduce the size of the paste cutout area in the stencil. In a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area. If you just reduce the paste opening aperture, providing one smaller opening, but don t segment it, you may end up with a part that is still too high in the middle to assure good contact on the signal pads and is also unstable and will likely tilt to one side. With leaded solder, a single 50% sized opening may work because of the wicking properties of lead-based solder. Since lead-free solder does not wick as well, it is very unlikely to work in a RoHS process. In both cases, the most consistently reliable method is to segment the stencil pattern. This is an example of recommended practice. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of highcenteringand other problems associated with large paste areas, such as out gassing and spattering. This will give good solder distribution with little chance of high-centering or outgassing problems. Specialized Copper Pad Some parts, especially high-frequency parts, require a segmented copper pad under the QFN. If this is the case, it is important to segment the solder paste stencil to match thecustom pad. It is fairly common practice to use a standard full-size square opening and hope that surface tension will end up distributing the solder in the right places. While that may happen, the chances of it not happening are equally great or greater. For best reliability and buildability, make sure that the openings match your copper layer underneath the stencil openings. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.Larger opposites With larger QFN parts, the opposite problem can occur in the center padarea. When the square opening for the solder paste stencil is fully open on a larger part say 10 x 10 mm or larger the paste squeegee may deform and actually scoop too much of the paste out of the opening. This can lead to uneven paste and solder voids. Both are potential reliability problems. The solution is the same. Segment the stencil opening to create an even paste distribution. SummaryThe QFN form factor delivers a number of advantages over other SMT package form factors. It is generally a smaller part and, with the center pad, can have better grounding and thermal properties. These advantages are partially offset by layout and assembly difficulties. But by following a few simple guidelines, you can use the parts with good confidence. Check the layout guidelines in the component applications notes. Segment your solder stencil opening for the center pad. Make a custom component library for your CAD package if you need to. Then Design away.
QFN Devices
QFN Layout GuidelinesWhile QFN (quad flatpack, no leads) and DFN (dual flatpack, no leads) packaged parts are becoming more and more common in new component releases, they aren t getting much easier to use. The advantages of the form factor are pretty clear. It allows smaller geometries, better grounding and improved thermal properties over other types of surface mount packages.
Most QFNs have a center metal pad on the underside of the part, typically for grounding or heatconduction. It s this center metal pad that makes this form factor so difficult to use. DFN packages are identical, for the purposes of assembly, except that they have pin rows on only two sides of the part.
QFN Float- The middle of the part has a metal contact pad like most QFN packaged parts. Itmay be there for grounding or heat conduction, depending on the specific part. Thefloat that I'm talking about happens when we lay too much solder paste on the PCBfor that center pad. To a small extent, the height of the solder paste deposit is proportional to theaperture in the solder stencil opening (bigger opening = taller deposit). With mostparts, that isn't a problem because either all of the pads are big enough so that thatratio doesn't have a first order impact, or because all of the pads are the same sizeand will be equally impacted. Since the QFN center pad is a much larger opening in the stencil than the signal pad openings, and the signal pad openings are in the 10 - 20 mil or less range, this deposit height to width ratio will have a firstorder impact. When the opening for the center pad on the QFN is too large, the paste deposit in the center will be taller than the deposits on the small signal pin pads. The part high-centers and never gets the opportunity to contact the signal pads. In some cases, the part will tilt a little sideways and contact some of the signal pads but not all. Solder Paste Stencil Typically, the signal pads should have a standoff height of 2 - 3 mils after assembly. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from connecting. To help prevent this, the solder stencil opening should be broken into a series of smaller openings and should cover between 50 and 75% of the pad area. This means that when you lay out your PCB, you need to look carefully at the solder paste layer for your QFN components. If the solder paste layer in the CAD package part library just follows the copper pad pattern or the solder mask opening, you may need to customize the CAD package part library to avoid leading yourself into trouble. To better illustrate the proper way to make your solder paste stencil for QFN parts, I went to our back room and took a couple of photos of good and bad solder paste stencil practices. This is what a just about worst-case stencil would look like. Actual size for this part is 7 x 7 mm. Note how much surface area that the center pad has compared to the row of side cutouts. With most SMT components, it is standard procedure to reduce the size of the paste cutout area in the stencil. In a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area. If you just reduce the paste opening aperture, providing one smaller opening, but don t segment it, you may end up with a part that is still too high in the middle to assure good contact on the signal pads and is also unstable and will likely tilt to one side. With leaded solder, a single 50% sized opening may work because of the wicking properties of lead-based solder. Since lead-free solder does not wick as well, it is very unlikely to work in a RoHS process. In both cases, the most consistently reliable method is to segment the stencil pattern. This is an example of recommended practice. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of highcenteringand other problems associated with large paste areas, such as out gassing and spattering. This will give good solder distribution with little chance of high-centering or outgassing problems. Specialized Copper Pad Some parts, especially high-frequency parts, require a segmented copper pad under the QFN. If this is the case, it is important to segment the solder paste stencil to match thecustom pad. It is fairly common practice to use a standard full-size square opening and hope that surface tension will end up distributing the solder in the right places. While that may happen, the chances of it not happening are equally great or greater. For best reliability and buildability, make sure that the openings match your copper layer underneath the stencil openings. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.Larger opposites With larger QFN parts, the opposite problem can occur in the center padarea. When the square opening for the solder paste stencil is fully open on a larger part say 10 x 10 mm or larger the paste squeegee may deform and actually scoop too much of the paste out of the opening. This can lead to uneven paste and solder voids. Both are potential reliability problems. The solution is the same. Segment the stencil opening to create an even paste distribution. SummaryThe QFN form factor delivers a number of advantages over other SMT package form factors. It is generally a smaller part and, with the center pad, can have better grounding and thermal properties. These advantages are partially offset by layout and assembly difficulties. But by following a few simple guidelines, you can use the parts with good confidence. Check the layout guidelines in the component applications notes. Segment your solder stencil opening for the center pad. Make a custom component library for your CAD package if you need to. Then Design away.
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